1. Field of the Invention
The invention relates to an exposure calculating device which is used for a camera having a function of locking the measured brightness information (referred to as AE lock hereinafter).
2. Description of the Prior Art
A conventional device for exposure calculation is disclosed in U.S. Pat. No. 4,412,730, where the field of view about to be photographed is divided into a plurality of areas, and then the luminance values on each divided areas are measured. The difference between the maximum luminance value and the minimum luminance value is obtained, or the luminance at the center of the field of view is compared with the maximum or minimum luminance value to judge the conditions of photography. The luminance values derived from the areas are used to perform exposure calculation according to the conditions.
A so-called AE (automatic exposure) lock technique has also been known. According to this technique, the range of measured brightness values is adjusted to a desired object. The data about the luminance obtained from the desired object is stored in a memory. After the range is altered, exposure calculation and control operation are performed, based on the stored data about the luminance.
When exposure is controlled using the calculation method discussed in the aforementioned U.S. Pat. No. 4,412,730, the probability that the filed of view to be photographed, especially the main subject, provides an appropriate exposure is undoubtedly much higher than the conventional case where exposure is controlled using the average luminance of the field of view. However, it is believed that the main subject is not yet sufficiently taken into account. For example, when such an exposure calculation is performed, if the maximum luminance obtained on the measured, divided areas is 9 or more in APEX system, the luminance of the main object is set equal or nearly equal to the minimum luminance. However, the main object does not always exist in the area giving the minimum or nearly minimum luminance.
When the aforementioned AE lock procedure is employed, the luminance of the object is stored in a memory and so an appropriate exposure is assured with respect to the object. Unfortunately, the other brightness or darker areas may yield under-exposure or over-exposure.
In known cameras, a flashlight control device for controlling the amount of flashlight used for flashlight photography is incorporated either in the body of an electronic flash device or in the body of a camera. In either case, the area subjected to brightness control covers substantially the whole field of view to be photographed or a relatively broad region including the center of the field of view.
In these flashlight control devices, the brightness control level at which emission of flashlight is stopped is determined based on the ratio of the amount of reflected light to the given amount of emitted flashlight. As an example, an object having a certain reflectivity (average reflectivity) is disposed opposite to a flash device so as to include the area subjected to brightness control. Then, a flashlight is emitted toward the object. The brightness control level is determined based on the amount of light reflected from the object such that the object provides a suitable exposure.
In general flashlight photography, the amount of light reflected from an object to be photographed varies. Hence, over-exposure or under-exposure may take place, depending on the setting of the brightness control level. More specifically, if objective lens is changed to one having different focal length is used or the object consists of a single or plural persons, i.e., if the object accounts for a different proportion, then the amount of light reflected from the subject and falling on the flashlight device will necessarily varies. This changes the amount of flashlight, which is determined by the amount of light incident on the flashlight device. As a natural consequence, exposure is not stable.
In order to solve these problems, a means may be contemplated in which the area used for control over luminance is narrowed into a spot, which is directed to the object to be photographed, for luminance control. If this means is carried out, the area for luminance control is always placed within the object, and the amount of reflected light is maintained constant. As a result, an appropriate exposure will be obtained. If the object lies outside the area for luminance control, however, the amount of flashlight is made inappropriate with respect to the object, leading to unsuitable exposure. As the area for luminance control is decreased, the probability of this undesired situation increases.
An object of the present invention is to provide an exposure calculating device capable of correctly measuring the brightness of the main object even if an in-focus condition is obtained after a manual AE lock.